If a plane—two-dimensional—image is displayed on a surface, then every point of the surface emits or reflects light with approximately same intensity (and color) in all directions. This is the working principle of a traditional picture, like a postcard (reflection) or a traditional TV-image (light emission). In the case when a three-dimensional image is presented, the emitted light has a different intensity (and color) in the different directions, even if it is emitted from the same point. We may regard in this way a window pane or a hologram as a display. Hence, in order to display a three-dimensional image, there is needed a light emitting surface where the intensity (and color) of the light emitted from a single image point (pixel) may be controlled as the function of the emission angle (exit angle), with other words, the intensity of the light emitted in the different directions may be controlled.
With some of the known systems suitable for displaying spatial (stereoscopic) images, two images are projected, which may be separated from each other by color filters, polarising filters, or by time-sequentially driven eyeglasses. These effects of the separated images are perceived as three-dimensional, when the two images are sensed by the left and right eye, respectively. These images are not true three-dimensional images, because they only provide the same two perspectives, independent of the position of the viewer in relation to the image. There are other known devices, called autostereoscopic devices, which permit the viewing of stereo images also without aiding means. Such a stereoscopic display is disclosed in EP 0 721 132 and EP 0 729 055, among others.
In order to produce true or realistic three-dimensional images, a large number of light beams must be projected in the different directions in space, with the appropriate intensity/colour, which allow the viewer to see different perspectives from different viewpoints. In some of the prior art displays, two surfaces are used for displaying realistic three-dimensional images. The first front surface is a surface with a controllable light transmission or reflection, and the second back surface is an illuminating surface comprising light sources. One point of the back surface and one point of the front surface defines unequivocally a direction. With a possible embodiment, the image is created on the back surface by controlling the intensity and/or colour of the light sources, while on the first surface only masking is performed according to the selected viewing directions, by switching the image pixels on and off. With an other possible embodiment, the light sources on the back surface are continuously on, or they are only switched on or off, while the controlling according to the image information is made on the first surface. The first surface comprising the image pixels with controllable light transmission or reflection is preferably an LCD display.
Such solutions utilising an LCD display are disclosed, among others, in the documents U.S. Pat. No. 5,831,765, U.S. Pat. No. 5,036,385, WO 99/07161, EP 0 316 465 and U.S. Pat. No. 5,132,839. In these known solutions, illuminated strips are used behind an LCD screen, and the light of the strips are either transmitted or blocked by the controlled image pixels of the LCD screen.
In the solution disclosed in EP 0 316 465, there is an illuminated line behind every pair of LCD-pixel columns, and the light of the line passes through either one column or the other, corresponding to the control of the LCD pixels. This arrangement allows the display of a stereoscopic image with two viewing directions, but the resolution of the LCD-display is low, because two LCD-pixels are needed for an image point. The description suggests increasing the number of LCD-pixels associated to one illuminating line, in order to increase the number of viewing directions, but this leads to a further lowering of the resolution.
With an other possible embodiment, it is suggested to use one illuminating line (light source) behind each LCD-pixel column. In this case every pixel is illuminated by multiple light sources, which results in several viewing directions, having independently controllable light emissions in the same image point. Such a display is described in the publication “A prototype flat panel hologram-like display that produces multiple perspective views at full resolution”, by J. Eichenlaub, in: Proceedings of the SPIE Vol. 2409, pp. 102–112. The principle of this known solution is shown in FIG. 1. Here, the number of the light sources is essentially equal to the number of the image pixels in a row. Therefore, in order to produce an image with an acceptable resolution, a large number of very small light sources are needed. These light sources are extremely expensive, due to their small size and the large quantity needed. The light sources may be manufactured by optical methods (e. g. cylindrical lens matrix, disclosed in WO 94/06249), but this requires again a very precise and costly technology, and the illumination angle is also limited. A further disadvantage of this approach is the limited intensity which may be achieved. A similar method is disclosed in U.S. Pat. No. 5,036,385., but only for use as a stereoscopic display, i. e. with only two different views for the left and right eyes for the observer.
U.S. Pat. No. 5,132,839 teaches a solution where an appropriate optical system positioned between the illuminating surface and the LCD screen produces light beams in different directions, but parallel to each other. With this system the LCD-screen is illuminated periodically in different directions, and the LCD-screen is controlled such that the image corresponding to the actually illuminated direction should appear on the on the LCD-screen in the corresponding moment. This solution also requires the use of small light sources, which results in the low intensity, as mentioned above. Also, the optical system (Fresnel lens, lens matrix) for the parallel illumination of the LCD displays makes the device expensive and complicated.
Accordingly, it is an object of the invention to provide a method and apparatus for the presentation of realistic three-dimensional images, preferably for moving images, which allows the display of the images without aiding means and without spatial limitations, and which further do not need expensive focusing and deflecting optical elements.